Fix build with dune 3.6

Releasing dune 3.6 revealed a regression on hdr_histogram (it builds
with 3.5 but not 3.6). After investigating, it seems to be an issue with
the dune file in hdr_histogram: `:standard` was missing from the
`(c_flags)` field. Adding it ensures this build with 3.6 too.

References:

- <ocaml/opam-repository#22498>
- <ocaml/dune#6481>

Author: emillon

lib/dune

@@ -12,7 +12,7 @@
   (vendored
     ; hack: multiple -I directives to work around cc commands being run from
     ; different relative directories.  Is there a cleaner way to do this?
-    (c_flags ("-Ilib" "-I."))))
+    (c_flags :standard "-Ilib" "-I.")))
   (headers (include "hdr_histogram.h"))
   (type_description
    (instance Type)